Sub-ambient cooling is conventionally accomplished through gas/liquid vapor phase compression based refrigeration cycles using Freon type refrigerants to implement the heat transfers. Such refrigeration systems are used extensively for cooling human residences, foods, and vehicles. Sub-ambient cooling is also often used with major electronic systems such as mainframe server and workstation computers. Though vapor compression cooling can be very efficient, it does require significant moving hardware, including at a minimum, a compressor, a condenser, an evaporator, and related coolant transfer plumbing. As a result of the complexity, associated high cost, and lower reliability, vapor compression cooling has not found material acceptance in small cooling applications, for example personal computers.
The fact that CMOS logic can operate materially faster as the temperature decreases has been well known for at least ten years. For example, if CMOS logic devices are operated at -50.degree. C., the performance is improved by 50 percent over room ambient temperature operation. Liquid nitrogen operating temperatures, in the range of -196.degree. C., have shown 200 percent performance improvements. Similar benefits have shown to accrue for integrated circuit wiring, where metal wiring resistances decrease by a factor of 2 for integrated circuits operated at -50.degree. C. in comparison to room ambient operation. This improvement rivals the recent technological breakthrough of using copper wiring in integrated circuits to reduce interconnect resistance and thereby effectively increase the operating frequencies attainable. Thus, sub-ambient operation of integrated circuit logic devices, such as field effect transistors, as well as the interconnect wiring can materially improve the integrated circuit performance, leaving the question of how to accomplish such cooling in the confines of an ever decreasing size and materially shrinking cost environment.
Thermoelectric cooling is one alternative that has found some usage given the compact size of the prevalently used Peltier devices. A Peltier device is fabricated from semiconductor material such as bismuth telluride or lead telluride. Though new materials are now being evaluated in various universities, they have yet to reach fruition. The commonly used Peltier materials exhibit very high electrical conductivity and relatively low thermal conductivity, in contrast to normal metals which have both high electrical and thermal conductivity. In operation the Peltier devices transport electrons from a cold sink, at temperature T.sub.cold, to a hot sink, at temperature T.sub.hot, in response to an electric field formed across the Peltier device.
FIG. 1 schematically depicts a conventional Peltier type thermoelectric element (TE) 1 with DC power supply 2 creating the electric field across TE 1 while at a load current 3. The desired heat transfer is from cold sink 4, at temperature T.sub.cold, to hot sink 6, at temperature T.sub.hot. As indicated in the equation of FIG. 1, the net heat energy transported is composed of three elements, the first representing the Peltier effect (thermoelectric) contribution, the second defining negative Joule heating effects, and the third defining negative thermal conduction effects. The thermoelectric component is composed of the Seebeck coefficient, the temperature of operation (T.sub.cold) and the current being applied. The Joule heating component reflects that roughly half the Joule heating goes to the cold sink and remainder to the hot sink. Lastly, the negative component attributable to thermal conduction represents the heat flow through the Peltier device, as defined by the thermal conductivity of the Peltier device, from the hot sink to the cold sink. See equation (1). EQU q=aT.sub.cold I-FR/2-K.DELTA.T (1)
While the reliability of thermoelectric cooling is better than mechanical-based cooling systems, there are a variety of long term degradation mechanisms which reduce the reliability of thermoelectric cooling below acceptable standards for thermoelectric elements operating at relatively high wattage. For example, single stage thermoelectric elements can only attain mean-time-between failure (MTBF) ratings of greater than 250,000 hours if T.sub.hot is kept below 90.degree. C. One of the primary degradation mechanisms leading to relatively low reliability thermoelectric cooling is the degradation of the junction between the semiconductor materials used to form the thermoelectric element and the materials used for the hot sink.
Typically, thermoelectric cooling systems connect several Peltier devices electrically in series and thermally in parallel making the thermoelectric cooling system vulnerable to a single Peltier device failing. The Peltier devices are typically created using copper tabs to create the electrical junctions to the semiconductor materials used to form thermoelectric element and thermal junctions to the thermal sinks used on the hot and cold end of the thermoelectric cooler. Degradation of the thermoelectric cooler occurs at the hot end of the cooler due to the solder used to create the junction between the copper tabs and the semiconductor materials. Bismuth-Tin alloy solders, containing small amounts of various metals such as tin, selenium, tellurium, antimony and nickel, are used to create the junctions between the copper tabs and the semiconductor materials. During operation, high levels of current flow through the thermoelectric cooler leading to a degradation mechanism at the hot end caused by electromigration. As high levels of current flow, the metals from the solder migrate to the semiconductor material creating metal spikes that penetrate the surface of the semiconductor material causing swelling and cracking of the solder joints leading to failure of the thermoelectric cooler.
Several configurations have been deployed to address the reliability of Peltier devices. One configuration, described in U.S. Pat. No. 5,650,904, uses a redundancy scheme that allows a failed Peltier device to be bypassed to avoid failures caused by Peltier devices electrically connected in series. The redundancy scheme suffers from performance loss when the redundant Peltier device and circuitry are activated. Though this design may increase the MTBF of the thermoelectric cooler, the overall cost and size of thermoelectric cooling, its efficiency and performance suffer due to the additional Peltier and bypass devices.
Thus, there are a number of very fundamental constraints on current thermoelectric coolers that have created a need for providing thermoelectric coolers having an increased reliability with relatively greater mean time between failures without compromising performance or efficiency.